Dehumidification of moist gas mixtures is necessary in a multitude of technical fields. For instance, ventilation and air conditioning of buildings or vehicles generally necessitates not only cooling but also dehumidification of the air since the air to be cooled is often so humid that during cooling to the desired temperature the temperature falls below the dew point temperature. Hence in conventional air conditioning systems dehumidification of the air accounts for a large part of the electricity consumption.
The electricity consumption of air conditioning systems for buildings is reduced by dehumidifying the air by adsorption or absorption of water with a drying medium and subsequently regenerating the water-laden drying medium by heating to a temperature at which the water is desorbed again. Compared to adsorption on a solid adsorbent, the advantage of absorption in a liquid absorption medium is that air dehumidification can be carried out with reduced equipment complexity and with less drying medium and that regeneration of the water-laden drying medium using solar heat is easier to carry out.
A further technical field where dehumidifying of moist gas mixtures is employed is the field of absorption chillers (also known as “absorption heat pump”, principle described in WO 2014/079675 A1). Here, the moist gas mixture is formed during evaporation of water under low pressure. The water vapour thus formed needs to be removed from the moist gas mixture so that said mixture may then be returned to the water evaporation to pass through a new cycle. Here too, absorption in a liquid absorption medium is favoured over adsorption on a solid adsorption medium.
Finally, dehumidification of moist gas mixtures is important in the field of natural gas extraction, as described in DE 10 2010 004 779 A1.
Examples of the materials typically incorporated in air or natural gas dehumidifying plants and in chillers include titanium, copper and noble metals.
The aqueous solutions of lithium bromide, lithium chloride or calcium chloride hitherto employed as liquid absorption media in commercial air conditioning systems have the disadvantage that they are corrosive towards the metallic materials of construction typically employed in air conditioning systems and that they thus necessitate the use of expensive special materials of construction. This problem is also encountered with steel and stainless steel. These solutions can additionally cause problems due to salt crystallizing out of the absorption medium.
Y. Luo et al., Appl. Thermal Eng. 31 (2011) 2772-2777 proposes using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate in place of aqueous solutions of lithium bromide for air dehumidification. However, this ionic liquid has the disadvantage of only poor absorption capability.
Y. Luo et al., Solar Energy 86 (2012) 2718-2724 proposes the ionic liquid 1,3-dimethyimidazolium acetate as an alternative to 1-ethyl-3-methylimidazolium tetrafluoroborate for air dehumidification. However, 1,3-dimethyimidazolium acetate is not stable and decomposes to a not inconsiderable extent during desorption.
This problem is also encountered for the ionic liquids proposed in US 2011/0247494 A1, paragraph [0145]. This document proposes using trimethylammonium acetate or 1-ethyl-3-methylimidazolium acetate as liquid drying agent in place of aqueous lithium chloride solution. Example 3 compares water uptake from humid air for a series of further ionic liquids.
CN 102335545 A describes aqueous solutions of ionic liquids that do not suffer from the abovementioned problems as absorption media for air dehumidification. CN 102335545 A chiefly describes steel-based air dehumidifiers. Ionic liquids described are, inter alia, 1,3-dimethyimidazolium dimethyl phosphate, 1-ethyl-3-methylimidazolium dimethyl phosphate and 1-butyl-3-methylimidazolium dimethyl phosphate. The absorption media are reported to be noncorrosive toward steel. However, heat transfer, which is important for efficient air dehumidification, is relatively low with the ionic liquids cited in CN 102335545 A in steel-based apparatuses.
Yet, heat transfer is an important parameter which must be accounted for when choosing absorption media. Thus, in the field of air dehumidification absorption media which ensure a particularly good heat transfer between themselves and further constituents of an air dehumidifier are particularly readily employable. In air dehumidifiers in which metallic components (for example steel) are employed this heat transfer takes place at least partially between the absorption media and metallic surfaces.
The present invention accordingly has for its object the provision of absorption media that ensure better heat transfer than prior art absorption media when used in steel-based air conditioning systems, air dehumidifiers, absorption chillers etc. Absorption media which achieve this object have now surprisingly been found.